Let's do some math ..  Rounding examples of 7 random hypothetical purchases:
  Case Tag  
Price  
Sales  
Tax %  
Raw Total  
Incl. Sales Tax  
Rounded  
to Penny  
Rounded  
to Nickel  
Rounded  
to Dime  
  Purchase #1 $246.86   3.728%   $256.0629   $256.06   $256.05   $256.10  
  Purchase #2 $40.57   9.256%   $44.3252   $44.33   $44.35   $44.30  
  Purchase #3 $39.59   13.157%   $44.7989   $44.80   $44.80   $44.80  
  Purchase #4 $31.82   11.204%   $35.3851   $35.39   $35.40   $35.40  
  Purchase #5 $166.40   10.130%   $183.2563   $183.26   $183.25   $183.30  
  Purchase #6 $186.42   10.657%   $206.2868   $206.29   $206.30   $206.30  
  Purchase #7 $82.53   3.416%   $85.3492   $85.35   $85.35   $85.30  
  7 purchase totals = $794.19       $855.4644   $855.48   $855.50   $855.50